The present invention relates to abrasion-resistant fluorocarbon polymer composites, such as polytetrafluoroethylene ("PTFE") composites, having high frictional efficiency over a wide range of temperature and load conditions. More particularly, the present invention relates to abrasion-resistant, anti-friction tubing.
Fluorocarbon polymers, such as PTFE resins, are well known in the art and have heretofore been utilized in extruded tubular products. Although PTFE resins in their pure form exhibit excellent frictional efficiencies, they generally have unacceptably low abrasion resistance, that is, they wear too rapidly. Attempts have been made to improve the abrasion resistance of PTFE resins by the addition of fillers, both inorganic and organic.
The wear resistance of PTFE extruded tubular products has traditionally been enhanced by the inclusion of inert, inorganic fillers such as glass fibers, carbon, asbestos fibers, mica, metals and metal oxides. See, for example, U.S. Pat. No. 3,409,584. While a measure of improvement in wear resistance has thus been achieved, PTFE composites comprising inorganic fillers nevertheless have several disadvantages. For example, such composites generally exhibit rapid deterioration in frictional efficiency after relatively short periods of use. Moreover, the use of such composites as liners for externally lubricated push-pull cable assemblies is not generally recommended because the inorganic fillers have been found to separate from the composite and form an abrasive slurry with the lubricant. This abrasive slurry not only decreases frictional efficiency, but it can also cause catastrophic and rapid failure of the liner. As a practical result, therefore, it has previously not been possible to successfully use inorganically filled PTFE composites in lubricated push-pull cable assemblies.
Fluorocarbon polymers have also been modified to include organic fillers. See, for example, U.S. Pat. Nos. 3,652,409 and 4,362,069. Generally, such organically filled fluorocarbon polymers, and particularly those filled with polyamide resins, do not lend themselves readily to extrusion, being more adapted to molding techniques. On the other hand, it has been found that prior art polymeric composites found suitable for producing tubular products by extrusion generally suffer from early deterioration under severe temperature and load conditions.
U.S. Pat. No. 4,451,616, issued to Kawachi et al, discloses a process for the preparation of a composite comprising PTFE and an organic filler is disclosed. Kwachi et al teach the use of a filler selected from the group consisting of polyimide resins, polyamide-imide resins, polyamide resins and carbon fiber powders. The Kawachi process involves coagulation of PTFE and one of the above mentioned fillers from an aqueous dispersion of these two components. The weight proportion of PTFE and the filler in their aqueous dispersion is disclosed as being from 100:5 to 80. Although the patent discloses that the abrasion resistance of PTFE can be enhanced by the incorporation of the above mentioned fillers, there is no indication that any one of those fillers is preferred over another, or that a particular concentration of filler in the composite is preferred.
U.S. Pat. No. 3,391,221, issued to Gore et al, discloses fluorocarbon polymer molding compositions containing from about 10 to about 55 volume percent of what are called "permanent lubricant modifiers" selected from the class consisting of (a) nonvolatile liquids which remain thermally stable and liquid at the sintering temperatures of the fluorocarbon polymer, and have lower vapor pressures at those temperatures and (b) materials which are liquid during the forming of the fluorocarbon polymer article and are transformed into a solid in the final shaped article. One important function of the lubricant modifiers of Gore is to act as a lubricating agent during shaping of the polymer. A variety of materials are disclosed as lubricant modifiers, including: aromatic polyamides formed by the reaction of aromatic dicarboxylic acids such as terephthalic acid with aromatic amines such as phenyl diamine or biphenyl diamine; the aromatic polyimides formed by the reaction of such acid dianhydrides as pyromellitic dianhydride with the stated aromatic diamine; the polyamide, polyimide copolymers from the above named components; aromatic polyesters formed from the aromatic dicarboxylic acids and aromatic diols; polybenzimidiazoles formed from the aromatic tetracarboxylic acids such as pyromellitic acid and aromatic tetramines; aromatic polyethers; and Novolac epoxy resins. The only guidance that the patent provides with respect to the selection of modifiers for the enhancement of frictional efficiency is that phenyl silicone lubricants are said to provide high lubricity under high unit loads, and that polymerizable monomers and prepolymers that are polymerized in situ provide molded articles that have a low coefficient of friction. The patent provides no indication that any particular concentration of filler is preferred over another.
U.S. Pat. No. 3,356,759, issued to Gerow, discloses compositions of aromatic polypyromellitimides and a polyfluorocarbon resin. Although this patent broadly refers to the presence of from about 10 to about 90% by weight of fluorocarbon resin in the composite, it expressly teaches that the composite preferably have no more than 50% by weight of the fluorocarbon resin. Accordingly, the Gerow reference teaches composites in which the polyfluorocarbon components preferably constitute a minor proportion of the composite.
Composites comprising a mixture of PTFE and polyarylene sulfide have heretofore been used in fabricating flexible liner or tubing for push-pull cable assemblies. For example, U.S. Pat. No. 4,362,069, issued to Gitras and assigned to the assignee of the present invention, describes a fluorocarbon composite fabricated from a mixture of PTFE resin and a polymer of arylene sulfide. The composite described in this patent has exceptional anti-friction, anti-abrasion characteristics across a relatively wide range of load and temperature conditions. However, as explained in the brochure entitled Abrasion Resistant Anti-friction Tubing in Push-pull Assemblies by the Markel Corporation, these composites must be used with an external lubricant in order to realize a performance advantage over unfilled PTFE products. When used without an external lubricant, these composites exhibit performance characteristics that are no better than conventional unfilled PTFE. Such composites not only have the disadvantage of requiring a lubricant, but also of precluding the use of significant amounts of inorganic filler in the organically filled composite. See, for example, col 5, lines 8-33 of the Gitras patent described above.
The compositions of the present invention are particularly well adapted for use as liners in push-pull cable assemblies and the like. Push-pull cable assemblies are typically used for the transmission of force or other mechanical control commands from one location to another in apparatus such as automobiles, aircraft, motorcycles, boats and bicycles. Such cable assemblies typically comprise a wire cable for transmitting the appropriate force and an abrasion-resistant, anti-friction liner surrounding the wire cable. Since the anti-friction, abrasion-resistant liner is the primary bearing surface in push-pull cable assemblies, it is subjected to unidirectional, reciprocating and/or rotary contact with the internal wire cable. In order to achieve superior or even acceptable liner life under these conditions, push-pull cable assemblies have heretofore typically required the application of lubricant between the liner and the wire cable.